1. Field of the Invention
The present invention relates generally to display devices using a material that exhibits electrogenerated chemiluminescence, also known as electrochemiluminescence (ECL), and a material exhibiting electrochromic (EC) phenomena.
2. Related Art
Half-transmissive liquid crystal display (LCD) devices offer both reflective displaying capability and light-emitting displaying capability, and are thus widely used as display panels of indoor/outdoor-use handheld electronic equipment, such as mobile cellular phones. Prior known half-transmissive LCD panels are designed so that a reflector layer with a convexo-concave surface is provided at part of a picture element or “pixel” for performing reflective display operations while a transmissive display unit is provided at the remaining area, and a backlight placed beneath this transmissive display unit, thereby to perform light-emitting display operations. One of the LCDs is disclosed, for example, in Published Japanese Patent Application No. 2003-241188, at pp. 3–5 and in FIG. 3 thereof.
In the case of such display panel, regarding the light-emitting display, a sufficiently bright and readily viewable display is achievable depending upon the brightness or luminosity of the backlight. Unfortunately, as for the reflective display, the display panel fails to offer eye-friendly display performances with enough contrast properties. This reason can be said because it suffers from limitations based on LCD's display principles, such as the use of a polarizer plate(s). Another reason is that each pixel is subdivided into two areas, one of which is exclusively used for reflective display, and the other of which is for light emission display, resulting in a decrease in effective display area.
Another approach to achieving a display device capable of performing high-contrast reflective display operations is to employ an electrochromic display (ECD) panel. The display device of this type is structured to have a pair of spaced-apart electrodes, between which disposed are an electrochromic (EC) material that is changeable in color due to electrochemical oxidation or reduction and an electrolyte. An example of it is found in Published Japanese Patent Application No. 2003-21848, at pp. 4–14, in FIG. 1. However, this ECD panel has the reflective displaying capability only, so its on-screen display images are difficult to be seen at dark places.
To ameliorate this problem, a display device capable of performing at a single pixel both the reflective and light-emitting display operations has been proposed, such as disclosed in Japanese patent application No. 2003-360535.
The reflective/light-emitting displayable panel as taught thereby is such that a reflective display operation is carried out by use of electrochromic (EC) phenomena with color changeability due to electrochemical oxidation and reduction or “redox,” whereas a light-emitting display operation is carried out by use of electrochemiluminescence (ECL) phenomena—that is, light emission takes place in the process of deactivation of an excited state created due to encounter or collision of ionic radicals, which have been created by electrochemical redox.
See FIG. 5, which illustrates in cross-section a display device of the type stated above. As shown herein, the display device includes a first substrate 11, a pair of first and second electrodes 16 and 17 which are provided on the first substrate 11, a second substrate 12 that is provided to be spaced apart from and oppose the first substrate 11, and a third electrode 13 provided on the second substrate 12 side. In addition, a second layer 14 is provided between the first and second substrate 11–12 at a location near the second substrate 12. The second layer 14 contains therein an EC material with color changeability due to electrochemical oxidation or reduction. Provided on the first substrate 11 side between the first and second substrates 11–12 is a first layer 15 which is made of an electrolytic material, in which is contained an ECL material that emits light through electrochemical oxidation or reduction. One set of the first to third electrodes 16–17 and 13 makes up a single pixel.
As the ECL material is contained in the first layer 15 (referred to as “ECL layer” hereinafter), when a voltage is applied thereto, this material is oxidized at part in close proximity to the electrode, resulting in creation of cation radicals (oxidation species); when this material is reduced, anion radicals (reduction species) are produced therein. Upon association of these cations and anions, an excited state of the ECL material takes place to emit light during its deactivation process. Using this phenomenon, light-emitting displaying is carried out.
Also note that as the EC material is contained in the second layer 14 (referred to hereinafter as “EC layer”), electrochemical oxidation or reduction occurs upon application of a voltage thereto. This results in a change in color, such as either color generation or color disappearance. Using this phenomenon, reflective display is performed.
When being instructed to perform the light-emitting display, an alternate current (AC) voltage is applied between the first and second electrodes 16–17. Alternatively, when being instructed to perform the reflective display, a direct current (DC) voltage is applied between the third electrode 13, and the first and second electrodes 16–17. The display device is used switching between these light-emitting and reflective display modes when the need arises.
It has been revealed that the display device having such the cell structure suffers from a problem: the ECL layer (i.e., first layer 15) decreases in efficiency of the ECL phenomenon. One cause of such reduction of ECL efficiency is that cation and anion radicals as created on or above the electrode(s) badly behave to diffuse or spread, resulting in deactivation prior to the association of these radicals.
One known approach to avoiding this problem is to employ a device structure which is designed to shorten the distance between the first and second electrodes 16–17 and the second layer 14, thereby lessening the volume in which the produced cation and anion radicals are spreadable. Unfortunately, this approach does not come without accompanying a penalty which follows. Highly reactive radical ECL molecules are made easier, by such spread, to reach the EC layer (second layer 14) and then come into contact with the EC layer, resulting in generation of secondary reactions, known as side reactions. This can deteriorate the EC layer in case the display device is used for an extended length of time period.